1. Field of the Invention
This invention relates to a method and an instrument for automatically determining the concentration of chlorine and chlorine dioxide in either one or both of a gaseous mixture or an aqueous solution.
2. Description of Prior Art
Chlorine dioxide is a commercially important material in such fields as pulp bleaching, water treatment and fat decoloring, and also has recently been used for the denitration of industrail waste gases and removal of phenols from industrial sewage.
One of the methods for generating chlorine dioxide is to reduce a chlorate with a reducing agent in a strong acid. The reactions which occur are exemplified below, wherein, for the sake of illustration, the chlorate used is sodium chlorate and the reducing agent is hydrochloric acid. EQU NaClO.sub.3 +2HCl.fwdarw.ClO.sub.2 +1/2Cl.sub.2 +NaCl+H.sub.2 O (1) EQU NaClO.sub.3 +6HCl.fwdarw.3Cl.sub.2 +NaCl+3H.sub.2 O (2)
Chlorine dioxide is formed by reaction (1), but not formed by reaction (2) which competes with reaction (1). It is well known in the field of this invention that the production of chlorine dioxide decreases as the chlorate-to-reducing agent molar ratio and the acid concentration of the reaction medium decreases. The respective concentrations of chlorine and chlorine dioxide in a gaseous mixture removed from a chlorine dioxide generator are determined so as to keep the production high and to prevent the chlorine dioxide from exploding.
Since chlorine dioxide is explosive and highly poisonous and, therefore, hazardous, it is generally absorbed by water and then is used as an aqueous solution in such fields as pulp bleaching and so on. In order to introduce a known controlled amount of chlorine and chlorine dioxide for a pulp bleaching plant, it is always required to determine the respective concentrations of chlorine and chlorine dioxide in such an aqueous solution.
The conventional, generally used procedure for determining the concentrations of chlorine and chlorine dioxide either in a gaseous mixture or in an aqueous solution is the iodimetric procedure in which iodine formed by reaction between iodide ion and chlorine or chlorine dioxide is titrated with sodium thiosulfate. This procedure, however, requires much manpower and much time, and sometimes it is not accurate. The need for a fully automatically controlled process for generating chlorine dioxide is being increasingly recognized these days. With the above in mind, an automatic process analyzer for continuously monitoring chlorine and chlorine dioxide in process fluids has been desired in order to generate and use chlorine dioxide without any danger of explosion and any loss of efficiency.